Under the Bayesian interpretation of probability, why is it be unfair for someone who isn't a player to secretly change the top card of the deck in a poker tournament? No player has gained any additional information about the deck, so it's still random to them.

Clearly a game of poker would still be fair if all the spectator did was look at the deck. So why does making a change (that none of the players can predict) become a problem?

The best answer I have at the moment is that if the change is known to the other spectators, it's changed the game for them because now the probabilities they assign to each player winning have changed away from a traditional game of poker.

But this implies that as long as the spectator makes the change secretly and no one else knows it happened, there's nothing unethical about that.